Fiber reinforced plastics are composite materials consisting of a polymer matrix reinforced with fibers like glass, carbon, or plastic. A polymer without fiber reinforcement can be relatively weak. Many plastic articles are reinforced with fibers to improve strength, rigidity, impact resistance and other physical properties. Factors determining the desired properties include fiber length and the distribution of lengths in the fiber population. Therefore, it can be important to persons involved in the manufacture of such fiber reinforced articles to determine the distribution of fiber lengths in a given product. This can be done by recovering the fibers from a product sample and determining the fiber length distribution in the fiber population.
It is known to sort fibers according to length through the use of sieves of a woven construction as shown, see for example, U.S. Pat. No. 6,925,857 ('857) the entire disclosure of which is incorporated herein by reference. The sieves in that patent are designed to collect and sort fibers according to size. In the '857 disclosure, a sample composite article is first heated to “burn-off” the polymer leaving behind the reinforcing fibers. The reinforcing fibers are then grouped according to their length using a fiber separator. The fiber separator comprises a series of sieves each with a screen. The cross-section of a screen is constructed to retain fibers of a predetermined length, and to pass fibers smaller than that predetermined length to another sieve with a screen with still smaller screen openings. In this manner, longer reinforcing fibers are trapped by the uppermost coarse screen, while successively shorter reinforcing fibers are captured by the successively finer screens. Each sieve is weighed individually to calculate the distribution of the fiber lengths in the sample.
In operation, the fibers are suspended in a liquid, and the fiber solution is passed through the fiber separator. However, because the fibers are randomly oriented in the liquid, the accuracy of the sorting process is not optimum; i.e., longer fibers may pass through a sieve if oriented diagonally to a sieve opening while shorter fibers are caught by the same sieve.